Coello allowed one run over two innings on Saturday afternoon against Philadelphia, and along the way caught the attention of manager John Farrell.
"I thought Coello threw the ball really well last night," Farrell said. "When he pitches, he has always shown that he believes in himself. He has a lot of confidence in his fastball."
Coello has been promoted from the Minor Leagues twice this season, but has only made two appearances at the big league level. He has allowed just the two runs on five hits, while striking out six, in four innings of work entering Sunday.
The 27-year-old previously worked with Farrell in Boston, where he tossed 5 2/3 innings in six games. Since that time in 2010, the native of New Jersey has switched from a curveball to a slider, which has become one of the main reasons for his recent success.
"I think he has a natural cut to his fastball, but the ones that were in the 86-87 [mph] range, that's a little bit of a slider/cutter," Farrell said. "He's done a good job of bringing that along. Before, it was more of a show type curveball that he had and that funky forkball he has."
Coello isn't expected to enter the mix for Toronto's rotation, but he could find a home in long relief if Carlos Villanueva or Aaron Laffey are transitioned into a starting role.